1. Field of the Invention
The present invention relates to methods of fabrication of structural units from titanium alloy materials, and more particularly to methods for preventing blockage or closure of fluid conveying gas passageways communicating the interior, and provided at the periphery, of a multi-sheet stack of titanium alloy materials.
2. Background of the Invention
The family of titanium alloys, including titanium aluminide materials, have become well known in the metallurgical arts for their excellent high-temperature strength and oxidation and creep resistance characteristics.
Titanium aluminides, like other titanium alloys, are typically brittle and difficult to process and/or fabricate at or near room temperatures. Two fabrication techniques which have found widespread utility in the fashioning of structures for various industries are diffusion bonding (DB) and superplastic forming (SPF).
Diffusion bonding refers to the metallurigical joining of surfaces of similar or dissimilar metals by the application of heat and pressure for a time duration so as to cause a co-mingling of atoms at the joint interface.
Superplastic forming refers to a forming process in which a metal, under high temperature and pressure conditions, exhibit unusually high tensile elongations with minimal necking. Titanium alloys are one of only a few metals which exhibit these "superplastic" properties.
The fabrication of structural units made of multi-layered stacks of titanium aluminide materials is made easier by utilization of the metallurgical processes of diffusion bonding and superplastic forming. Multi-layer stacks generally comprise three or more stacked sheets of metal selectively stopped-off to facilitate bonding of only predetermined surface regions of facing sheets.
Prior to bonding, the stack of sheets is typically fitted with long (on the order of about 25 inches), slender, flexible gas-conveying tubing or "needles" at various peripheral locations for communicating the interior of the stack with a source of pressurized gas located outside the stack. These needles enable conveyance of the pressurized gas to the stack interior for effecting superplastic forming subsequent to carrying out the diffusion bonding step.
A serious problem has arisen which is addressed by this invention. Typically, the gas needles which have been mounted in openings located at the periphery of the stack extend outwardly from that periphery. During the diffusion bonding process, both the needles and the stack, although housed within a stainless steel protective retort, are subject to pressures of a diffusion bonding magnitude. While the principal aim of applying such pressures is to effect diffusion bonding of selected surfaces within the stack of sheets, the pressure is also sufficient to crush the needles along their entire length up to the periphery of the stack. As a result, it has been found necessary to replace the needles prior to effecting gas expansion during the subsequent superplastic forming step. This is terribly time consuming, expensive and inconvenient. In addition, there is a tendency for the passageways at the periphery of the stack within which the needles are fitted to close down.